Vectoring technology will be the core technology of the next generation of DSL. It cancels the FEXT (far-end crosstalk) between DSL lines, and therefore maximizes DSL system performance. Vectoring technology will play a very important role in FTTx (Fiber To The Node/Cabinet/Curb/Building/Home/Premises, etc.) business, because it enables offering 100 Mbps per user with DSL lines in the last hundred meters, i.e. between the end of a fiber network and the CPEs (Customer premises Equipments).
ITU-T (Telecommunication Standardization Sector of the International Telecommunication Union) has been standardizing a vectoring standard G.993.5 [1], and the first recommendation of G.993.5 was approved on Apr. 22, 2010. The cancellation of the FEXT is done at the DSLAM (Digital Subscriber Line Access Multiplexer) side. The downstream FEXT is pre-cancelled by a precoder in the DSLAM, while the upstream FEXT is cancelled by an upstream crosstalk canceller in the DSLAM. The recommendation provides a way to estimate the FEXT channel in both downstream and upstream and utilize the estimated channel to cancel the crosstalk.
A Disorderly Shutdown Event (DSE) on a DSL line occurs e.g. when a user unplugs the telephone cable or turns off the CPE abruptly. The disorderly shutdown of a DSL line may change the crosstalk channel characteristics, i.e. the crosstalk coupling to other lines, due to the impedance change at the CPE end which is disorderly shut down. After a DSE, the DSLAM is required, e.g. in VDSL2 (ITU-T G.993.2 [2]), to continue to transmit for about 10 seconds to the CPE which is disorderly shut down, see section 12.1.4 in [2]. The transmission is continued in order to avoid unnecessary retraining of a line at the occurrence of short interruptions of the line. In case the CPE “comes up” again within the 10 seconds, the transmission on the line may continue as before without retraining. Further, a time period of 2.5 seconds should elapse before it is established in the DSLAM that a power down-event has occurred.
However, when using vectoring, the precoder in the DSLAM remains unchanged after a DSE and continues to be optimized for the original channel characteristics, i.e. the channel characteristics before the DSE. This could result in a significant SNR (Signal to Noise Ratio) drop for other lines, since the precoder is outdated and thus cannot completely cancel the crosstalk from the line which is disorderly shut down. It has been reported in [3] that a DSE on one line can make other lines retrain. Retraining a line may take 30 seconds, which is a considerable interruption, e.g. in IP-TV services.
In the current standard G.993.5 [1], it is recommended that “the VTU-O (VDSL Transceiver Unit at the Optical Network Unit) switches off its transmit signal as soon as possible” after the detection of DSE in one line. It is further stated that “Other mechanisms for mitigating the effect of a disorderly shutdown are for further study.” Furthermore, it is agreed in ITU-T Huntsville meeting in March 2010 that:                “a disorderly leaving (i.e., CPE disconnect or powerdown) event on a particular line should not cause a retrain on other lines in the same vectored group.”        “a disorderly leaving (i.e., CPE disconnect or powerdown) event on a particular line should not cause errored symbol sequences on other fines in the same vectored group that cannot be corrected by a valid/NP (Impulse Noise Protection) configuration.”        
Thus, the recommended solution in the current G.993.5 [1] is in conflict with the VDSL2 recommendation [2], which requires the VTU-O to continue to transmit the downstream signal on the line in question for about 10 seconds after a DSE on a line. Following the recommended solution in G.993.5 [1] may bring problems which are avoided when following the VDSL2 recommendation.
It is identified as a problem that a DSE or disorderly leaving event on a DSL line in a vectored group of DSL lines may cause other lines in the vectored group to retrain when the transmission on said DSL line is continued after the disorderly event. Since it is not desired to terminate the transmission on the line which is subject to a DSE, which would have been a straightforward solution, another solution should be sought.